1. Field of the Invention
The present invention relates to a rear-projection type screen employed in a rear-projection type television set.
2. Description of the Related Art
Examples of well-known rear-projection type screens of the prior art employed in rear-projection type television sets include, as shown in FIG. 10, a 2-sheet type in which a Lenticular lens sheet 1 and Fresnel lens sheet 2 are combined, and as shown in FIG. 11, a 3-sheet-type in which a Lenticular lens sheet 1, Fresnel lens sheet 2 and front plate 3 are combined. In these rear-projection type screens of the prior art—in which 2 or more plastic lenses of different thickness and material quality are combined—gaps, which have their origin in the difference in magnitude of the elongation and shrinking of the constituent members (lens sheets or front plate) with respect to changes in temperature and humidity of the surrounds, are generated between the constituent members. For this reason, rear-projection type screens (by way of example in Japanese Unexamined Utility Model Application No. Heisei 1-142901) have been configured in which, in the manufacturing stage of the lens sheets, warp is imparted in advance by the mounting of a flat surface lens sheet in a mold and the heating thereof in this state, the elastic forces of the lens sheets are utilized to closely attach the lens sheets, and the lens sheets are fixed in the screen frame in such a way that no gaps are formed.
When a television set comprising a rear-projection type screen, in which lens sheets and a front plate are fixed in the projection sheet frame, is transported for a long time, abrasion between the lens sheets occurs which is caused by vibration during transportation. As a result, the top of the Fresnel lens and the top of the Lenticular lens are pared, or the rear surface of the front plate is pared, whereby the problem of a significant reduction in television set image quality is generated. Generally, in the state in which the screen is being used, the paring of the top of the Fresnel lens and the like is generated mostly in the region of the upper and lower edge parts.
In order to solve this problem of the paring of the top of the Fresnel lens and so on, a screen in which the sharpened section of the top of the Fresnel lens is flattened (see Japanese Unexamined Patent Application No. Heisei 2-93531), and a screen in which the macroscopic cross-sectional shape of the Lenticular lens sheet is formed to approximate a spherical shape in which the curved lines and straight lines are combined (Japanese Unexamined Utility Model Application No. Heisei 1-97327), have been proposed.
However, in order for the image quality not to be reduced for screens in which the top of the Fresnel lens has been flattened, there are limits that only, a minute section of the top of the Fresnel lens, whose width at the top may be only several μm and through which the projected light does not pass must be flattened, while the shape of the other section through which the projected light passes must not be altered. For this reason, the prevention effect on the paring caused by the abrasion of the lens sheets is inadequate. In addition, in a Fresnel lens sheets for rear-projection type screens, the above-mentioned flat part of the top of the Fresnel lens which have concentric graves cannot be provided selectively in just the upper and lower edge regions of the screen through manufacturing process employing a metal mold for the lens sheet produced by cutting each single groove with a cutting tool. Thereupon, a problem arises in that the cutting must be performed to a region, from the left and right edges of the screen, in which the provision of the flat part in the top of the lens is unnecessary.
On the other hand, in a screen in which the macroscopic cross-sectional shape of the Lenticular lens sheet is an approximately spherical shape in which the curved lines and straight lines are combined, gaps are generated between both lens sheets in a region in which the macroscopic cross-sectional shape of the Lenticular lens sheet constitutes a straight line shape in environments of high temperature and the like. When the size of the warp is enlarged to solve this problem, a paring of the top and so on of the Fresnel lens is liable to occur due to the abrasion of the sheet at a region in which the macroscopic cross-sectional shape of the lens sheet constitutes a curved line shape. For this reason, it is difficult to remove the problems of elimination of the gap between the sheets and prevention of paring of the top and so on of the Fresnel lens simultaneously.
In addition, in order to prevent the paring of the top of the Fresnel lens and so on due to the abrasion caused by vibration during transportation, the attachment of a tape to the outer circumferential part of the lens sheets or front plate has been performed by operators. However, there are times when the lens is damaged during the application of the tape, because this operation is complicated.